Changes in the caliber (diameter) of the airways of the lung may accompany a number of respiratory diseases, and may have important consequences for a patient. An example of such a change is the airway narrowing that occurs during an acute attack of asthma, when a subject may experience great difficulty in breathing. During such occasions, the flow resistance of the airways increases substantially. Measuring this resistance thus gives a functional assessment of airway narrowing, and is therefore useful as a gauge of disease severity or the efficacy of treatment.
There have been many studies of ways to measure airway resistance in both humans and experimental animals. A standard clinical method for measuring airway resistance in humans includes having a subject perform special breathing maneuvers while inside a closed chamber called a plethysmograph. This type of procedure is often not possible with experimental animals and young children (e.g., those below about 5 years of age). The other standard clinical test of lung function is to have the subject take a deep breath and then exhale as forceably as possible. The amount of air exhaled in a set period of time is sensitive to the presence of certain lung diseases, but is only loosely related to actual airway resistance. This approach again is difficult in animals and young children, although it is employed in sedated infants by subjecting them to a whole-body squeeze designed to force air out of the lungs. In the case of animals, the standard approach is to measure airway resistance using invasive procedures that procure the necessary pressure and flow signals. These methods are accurate, but invariably either harm the animal or at the very least place it under unnatural circumstances that likely affect the resistance measurements obtained.
There is, thus, a great need for a means of measuring airway resistance that is both convenient and noninvasive, for use with all subjects (including young humans and experimental animals). Such a method currently does not exist, despite the widespread use of surrogates based purely on the measurement of the pattern of breathing (i.e., volume inspired per breath and rate of breathing).